1. Field of the Invention
This invention relates to an impeller used for cooling a dynamoelectric machine such as electric motors or generators, and more particularly to an improvement in the noise reduction in such an impeller.
2. Description of the Prior Art
With an increasing concern about environmental noise, various attempts have recently been made to reduce noise in dynamoelectric machines such as electric motors or generators. Particularly in a dynamoelectric machine provided with a cooling fan, most of noise produced in the machine results from noise produced by blades of an impeller of the fan. Accordingly, many attempts have been made to reduce the noise produced by the blades of the impeller.
Generally speaking, the noise produced with rotation of an impeller is distributed in a wide frequency range. When a plurality of blades are arranged radially on a disk at equal spaces, a noise with a predetermined frequency and an extraordinary high sound pressure level is produced. This noise is a periodic noise having a fundamental frequency f (Hz) obtained by multiplying the number of blades by a rotational frequency (Hz), namely, a noise with a frequency represented by 1.times.f, 2.times.f, 3.times.f, . . . This noise is referred to as "simple tone." A noise having the simple tone as a component is generally hard to hear and offensive to the ear.
To reduce the above-described periodic noise, Japanese Patent Publication No. 55-25555A proposes an impeller comprising a plurality of blades arranged at unequal spaces. In the proposed impeller, however, due consideration is not given to a mechanical balance. More specifically, FIG. 15 shows an impeller 1 for a propeller type axial flow fan disclosed in the above-referenced gazette. The impeller 1 comprises a disk 2 and five blades 3 arranged on an outer circumference of the disk 2 in a circle or radially. In FIG. 5, reference symbols L1 to L5 denote axes passing through the centers of gravity of the respective blades 3 and the center of rotation. As obvious from FIG. 15, the impeller 1 is symmetrical about each of the axes L1 and L2, whereas the impeller 1 is unsymmetrical about each of the axes L3 to L5. Further, angles between the blades 3 vary to a large degree, and a maximum angular difference is 66.9 degrees.
When the symmetry of the impeller 1 is collapsed to a large degree, centrifugal forces applied to the respective blades 3 during rotation of the impeller become unbalanced, whereupon there is a possibility of occurrence of vibration. Further, flows of cooling air supplied by the impeller are disturbed when the angular differences between the blades 3 become large. As a result, noise due to a turbulent flow of air is increased. Further, an amount of cooling air supplied by the impeller is reduced and accordingly, the cooling performance of the fan is reduced.